Dark skin
[5][6][7][8] The evolution of dark skin is believed to have begun around 1.2 million years ago,[9][10] in light-skinned early hominid species after they moved from the equatorial rainforest to the sunny savannas.
The loss of body hair led to the development of dark skin pigmentation, which acted as a mechanism of natural selection against folate (vitamin B9) depletion, and to a lesser extent, DNA damage.
As a consequence of vitamin D deficiency, they are at a higher risk of developing rickets, numerous types of cancers, and possibly cardiovascular disease and low immune system activity.
Studies into non-African populations indicates dark skin is not necessarily a retention of the pre-existing high UVR-adapted state of modern humans before the out of Africa migration, but may in fact be a later evolutionary adaptation to tropical rainforest regions.
[23][24][25] Due to mass migration and increased mobility of people between geographical regions in the recent past, dark-skinned populations today are found all over the world.
As hominids gradually lost their fur between 1.2 and 4 million years ago, to allow for better cooling through sweating, their naked and lightly pigmented skin was exposed to sunlight.
In the tropics, natural selection favoured dark-skinned human populations as high levels of skin pigmentation protected against the harmful effects of sunlight.
Genetic evidence also supports this notion, demonstrating that around 1.2 million years ago there was a strong evolutionary pressure which acted on the development of dark skin pigmentation in early members of the genus Homo.
It was harder to get food in the hot savannas and as mammalian brains are prone to overheating—5 or 6 °C rise in temperature can lead to heatstroke—there was a need for the development of better heat regulation.
Nina Jablonski, a professor of anthropology and expert on evolution of human skin colouration,[41] found several cases in which mothers' visits to tanning studios were connected to neural tube defects in early pregnancy.
[3][23] As humans dispersed from equatorial Africa to low UVR areas and higher latitudes sometime between 120,000 and 65,000 years ago, dark skin posed a disadvantage.
It is known that dark-skinned people who have moved to climates of limited sunlight can develop vitamin D-related conditions such as rickets, and different forms of cancer.
[28][52][53][54][55] However, these cancer types usually affect people at the end or after their reproductive career and could have not been the evolutionary reason behind the development of dark skin pigmentation.
It has been argued that the low melanoma mortality rates during reproductive age cannot be the principal reason behind the development of dark skin pigmentation.
[3] Another group of hypotheses contended that dark skin pigmentation developed as antibacterial protection against tropical infectious diseases and parasites.
Due to the heavily melanised melanosomes in darkly-pigmented skin, it can absorb more energy from UVR and thus offers better protection against sunburns and by absorption and dispersion UV rays.
[28] Despite this, medium-wave ultraviolet radiation (UVB) damages the immune system even in darker skinned individuals due to its effect on Langerhans cells.
[28] The stratum corneum of people with dark or heavily tanned skin is more condensed and contains more cornified cell layers than in lightly pigmented humans.
This disease in the past was commonly found among dark-skinned Americans of the southern part of the United States who migrated north into low sunlight environments.
Deformities of the female pelvis related to severe rickets impair normal childbirth, which leads to higher mortality of the infant, mother, or both.
[3][87][88] In addition, recent studies have linked vitamin D deficiency to autoimmune diseases, hypertension, multiple sclerosis, diabetes and incidence of memory loss.
Outside the tropics UVR has to penetrate through a thicker layer of atmosphere, which results in most of the intermediate wavelength UVB reflected or destroyed en route; because of this there is less potential for vitamin D biosynthesis in regions far from the equator.
[26][28] Evolution has restricted humans with darker skin in tropical latitudes, especially in non-forested regions, where ultraviolet radiation from the sun is usually the most intense.
[95] Natives of Buka and Bougainville at the northern Solomon Islands in Melanesia and the Chopi people of Mozambique in the southeast coast of Africa have darker skin than other surrounding populations.
According to Nina Jablonski, darkly-pigmented modern populations in South India and Sri Lanka are an example of this, having re-darkened after their ancestors migrated down from areas much farther north.
However, a 1999 study failed to find clear indications of a single shared genetic origin between the two populations, suggesting multiple waves of migration into Sahul with distinct ancestries.
These differences depend in part on general distance from the equator, illustrating the complex interactions of evolutionary forces which have contributed to the geographic distribution of skin colour at any point of time.
[43] Due to frequently differing ancestry among dark-skinned populations, the presence of dark skin in general is not a reliable genetic marker, including among groups in Africa.
For example, Wilson et al. (2001) found that most of their Ethiopian samples showed closer genetic affinities with lighter-skinned Armenians than with darker-skinned Bantu populations.
[113] Several genetic surveys of South Asian populations in different regions have found a weak negative correlation between social status and skin darkness, represented by the melanin index.
